Manchester codes (also variously known as biphase and phase modulation) are extensively used in the transmission of digital data. Their chief advantage is their self clocking feature; their chief disadvantage is that the channel must provide a bandwidth slightly in excess of the bit rate. There are two general approaches to decoding Manchester codes: the first is commonly known as an "integrate-and-dump" technique; and the second approach is a sequential operation where non-significant or phase bit transitions are masked in generating a strobe. Generally, the integrate and dump approach requires considerably more hardware.
In the integrate and dump technique, the signal is averaged over the bit period and a decision is then made as to the weight of the bit. A phase-locked-loop is usually used for the bit clock, and, when consecutive "1" or "0" bits are transmitted, the polarity of the received data is normally reversed during the second half of the bit period. The integrate-and-dump technique is considered to be a form of the "matched" or optimal filter approach to detection. In the integration, noise tends to cancel, leaving an integrated signal that is relatively pure.
As the bit frequency increases, it becomes more difficult to "dump" the integrator in a time interval that is insignificant compared to the bit period. Also, the polarity inversion requirement becomes increasingly more difficult to implement, as does the synchronization of the integrator to the bit clock.
In the sequential circuit technique, the usual operation of sequence recognition is extended. The necessity for recognizing the distinction between identical sequences that would give the same output except for a time-of-occurrence restriction (f vs. 2f rate) occurs. Thus, the task to be performed required more than counting and decoding.
A common approach today is creating a strobe to use a one-shot with pulse width greater than one-half the bit period which prevents the one-shot from clocking on the non-significant transitions. This technique, however, cannot provide maximum speed due to one-shot recovery time considerations.